Abstract The photolysis of ethylene carbonate vapour was studied at 206.2 nm at pressures from about 5 to 30 Torr and at temperatures from 120 at 160 °C. The major products were CO 2 and ethylene oxide, while the minor products were H 2, CO, CH 4 and CH 3CHO. The quantum yields for the production of CO 2 increased sharply at higher pressures, reaching values as high as 200, and there is evidently an unusual photo-initiated chain reaction occurring to produce CO 2, ethylene oxide polymer and smaller amounts of ethylene oxide. The thermal decomposition was also studied briefly between 150 and 500 °C; CO 2 and ethylene oxide were major products, apparently formed in a surface reaction. The photolysis of vinylene carbonate vapour around 220 – 240 nm at pressures from 1 to 5 Torr and a temperature of 60 °C gave CO and H 2 as the major products in a 3 to 1 ratio. Ketene and CO 2 were also observed in rather smaller yields. Mechanisms are discussed and absorption spectra are reported for both compounds.